Organic light emitting display

ABSTRACT

An organic light emitting display includes a display panel, which includes a plurality of pixels and displays an image, and a data driving circuit differently outputting a compensation voltage depending on a sensing value based on a driving current. Each of the plurality of pixels includes an organic light emitting diode, a driving thin film transistor (TFT) having a double gate structure including a main gate electrode and a sub-gate electrode, a switching TFT applying a data voltage determining the driving current to the main gate electrode of the driving TFT, and a compensation TFT applying the compensation voltage for compensating for a shift amount of a threshold voltage of the driving TFT to the sub-gate electrode of the driving TFT.

The present patent document is a continuation of U.S. patent applicationSer. No. 14/078,796, filed on Nov. 13, 2013, which claims the benefit ofpriority to Korea Patent Application No. 10-2012-0147751 filed on Dec.17, 2012, which is incorporated herein by reference for all purposes asif fully set forth herein.

BACKGROUND

1. Field of the Disclosure

Embodiments of the disclosure relate to an active matrix organic lightemitting display, and more particularly, to an organic light emittingdisplay capable of compensating for the degradation of a driving thinfilm transistor.

2. Discussion of the Related Art

An active matrix organic light emitting display includes organic lightemitting diodes (hereinafter, abbreviated to “OLEDs”) capable ofemitting light by itself and has advantages of a fast response time, ahigh light emitting efficiency, a high luminance, a wide viewing angle,etc.

The OLED serving as a self-emitting element includes an anode electrode,a cathode electrode, and an organic compound layer formed between theanode electrode and the cathode electrode. The organic compound layerincludes a hole injection layer HIL, a hole transport layer HTL, a lightemitting layer EML, an electron transport layer ETL, and an electroninjection layer EIL. When a driving voltage is applied to the anodeelectrode and the cathode electrode, holes passing through the holetransport layer HTL and electrons passing through the electron transportlayer ETL move to the light emitting layer EML and form excitons. As aresult, the light emitting layer EML generates visible light.

The organic light emitting display arranges pixels each including anOLED in a matrix form and adjusts a luminance of the pixels based on agray scale of video data. Each of the pixels includes a driving thinfilm transistor (TFT) controlling a driving current flowing in the OLEDbased on a gate-source voltage, a capacitor for uniformly holding a gatevoltage of the driving TFT during one frame, and a switching TFT storinga data voltage in the capacitor in response to a gate signal. Theluminance of the pixel is proportional to a magnitude of the drivingcurrent flowing in the OLED.

The organic light emitting display has disadvantages in that thresholdvoltages of the driving TFTs of the pixels are differently changeddepending on a formation position due to reasons of a process deviation,etc., or electrical characteristics of the driving TFTs are degraded dueto a gate-bias stress increased as a driving time passed. When theelectrical characteristics of the driving TFT are degraded, a currentcharacteristic curve of the driving TFT is shifted. Therefore, it isdifficult to achieve a desired luminance, and life span of the organiclight emitting display is reduced.

To solve these problems, in a related art, as shown in FIG. 1, after adeviation between electrical characteristics of driving TFTs of pixelsP, i.e., a deviation between threshold voltages of the driving TFTs ofthe pixels P is sensed by a driver integrated circuit (IC) DIC, aninternal operation is performed. A luminance difference resulting fromthe deviation between the threshold voltages is compensated by adjustingthe pixel data for the implementation of an image with reference to aresult of the internal operation.

For example, as shown in FIG. 2, when a positive stress is applied to agate electrode of the driving TFT for a long time to increase thethreshold voltage of the driving TFT from ‘Vth1’ to ‘Vth2’ by ‘φ’, and acurrent characteristic curve of the driving TFT is right shifted from‘A’ to ‘B’, a current flowing between drain and source electrodes of thedriving TFT is reduced from ‘I1’ to ‘I2’ by ‘ΔI’ under the sameconditions. In FIG. 2, ‘Vgs’ denotes a gate-source voltage of thedriving TFT. To compensate for a reduction in the current, the relatedart adopts a method for greatly modulating a data voltage applied to thegate electrode of the driving TFT by an increase amount of the thresholdvoltage while holding the current characteristic curve of the drivingTFT in a degraded state ‘B’. The positive stress the gate electrode ofthe driving TFT feels is proportional to a magnitude of an drivingvoltage as well as a length of an driving time. Thus, in the relatedart, a magnitude (Vth2+φ) of the data voltage applied to the driving TFThas to gradually increase, so as to compensate for the degradation ofthe driving TFT. As a result, as shown in FIG. 3, the degradation of thedriving TFT is accelerated in a compensation process.

Further, as shown in FIG. 4, a range of the voltage the driver IC DICcan output is previously determined depending on its object. Therefore,when a magnitude of a desired compensation voltage exceeds acompensation voltage range (i.e., 16V−12V=4V) of the driver IC DIC dueto the excessive degradation of the driving TFT, it is impossible tocompensate for the degradation of the driving TFT. The problem is causedbecause the degradation characteristic of the driving TFT is notsaturated at a time point but continued. Further, the problem is causedbecause the magnitude of the voltage capable of compensating for thedegradation of the driving TFT is limited.

The related art compensation method has the problems of the narrowcompensation range and a limitation of the compensation range, and thusit is difficult to solve the non-uniformity of the luminance and thereduction in the life span of the organic light emitting displayresulting from the degradation of the driving TFT.

SUMMARY

An organic light emitting display comprises a display panel including aplurality of pixels to display an image, and a data driving circuitconfigured to differently output a compensation voltage depending on asensing value based on a driving current, wherein each of the pluralityof pixels includes an organic light emitting diode, a driving thin filmtransistor (TFT) having a double gate structure including a main gateelectrode and a sub-gate electrode, a switching TFT configured to applya data voltage that determines the driving current to the main gateelectrode of the driving TFT, and a compensation TFT configured to applythe compensation voltage to compensate for a shift amount of a thresholdvoltage of the driving TFT to the sub-gate electrode of the driving TFT.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification, illustrate embodiments of the invention andtogether with the description serve to explain the principles of theinvention. In the drawings:

FIG. 1 schematically illustrates a connection relationship between adriver integrated circuit (IC) and a display panel;

FIG. 2 illustrates a related art method of compensating for thedegradation;

FIG. 3 illustrates that the degradation is accelerated by thecompensation in a related art method of compensating for thedegradation;

FIG. 4 illustrates an example of a range of a voltage a driver IC canoutput;

FIG. 5 illustrates an organic light emitting display according to anexemplary embodiment of the invention;

FIG. 6 illustrates a comparison between a compensation method accordingto an exemplary embodiment of the invention and a related artcompensation method;

FIGS. 7A to 8B illustrate a compensation principle of a thresholdvoltage of a driving thin film transistor (TFT);

FIG. 9 illustrates electrical characteristics of a double gate typedriving TFT;

FIGS. 10 and 11 illustrate kinds of a double gate type driving TFTcapable of performing a bidirectional control; and

FIGS. 12A to 12C sequentially illustrate a method of compensating for athreshold voltage according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Reference will now be made in detail to embodiments of the invention,examples of which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts. It will be paid attentionthat detailed description of known arts will be omitted if it isdetermined that the arts can mislead the embodiments of the invention.

Exemplary embodiments of the invention will be described with referenceto FIGS. 5 to 12C.

FIG. 5 illustrates an organic light emitting display according to anexemplary embodiment of the invention.

As shown in FIG. 5, an organic light emitting display according to anexemplary embodiment of the invention includes a display panel 10 onwhich pixels P are arranged in a matrix form, a data driving circuit 12for driving data lines 14 of the display panel 10, a gate drivingcircuit 13 for driving gate lines 15 of the display panel 10, and atiming controller 11 for controlling driving timing of the data drivingcircuit 12 and the gate driving circuit 13.

The display panel 10 includes the plurality of data lines 14, theplurality of gate lines 15 crossing the data lines 14, and the pluralityof pixels P respectively positioned at crossings of the data lines 14and the gate lines 15 in the matrix form. The display panel 10 furtherincludes sensing current supply lines SL (refer to FIG. 6) for sensing adriving current flowing in the pixels P and compensation voltage supplylines CL (refer to FIG. 6) for applying a compensation voltage to thepixels P. The gate lines 15 include scan signal supply lines forsupplying a scan signal SCAN (refer to FIG. 6), sensing control signalsupply lines for supplying a sensing control signal SEN (refer to FIG.6), and compensation control signal supply lines for supplying acompensation control signal CP (refer to FIG. 6).

As shown in FIG. 6, each pixel P includes an organic light emittingdiode (OLED), a driving thin film transistor (TFT) DT including two gateelectrodes, a switching TFT ST, and a first storage capacitor Cst1. Inaddition, each pixel P may further include a compensation TFT T2 and asecond storage capacitor Cst2. At least one of the pixels P may furtherinclude a sensing TFT T1. The driving TFT DT adopts a double gatestructure and includes a main gate electrode, to which a data voltagedetermining the driving current is applied, and a sub-gate electrode, towhich a compensation voltage for the compensation of a threshold voltageis applied. The sensing TFT T1 senses a current flowing in the drivingTFT DT, thereby sensing the shift of the threshold voltage of thedriving TFT DT. The sensing TFT T1 may be included in each pixel P, maybe included in each pixel group having at least two pixels P so as toincrease an emission area, or may be included in at least one of thepixels P. The compensation TFT T2 applies a compensation voltage 9 tothe driving TFT DT, thereby recovering the shift of the thresholdvoltage to an original state. The compensation TFT T2 may be included ineach pixel P. The second storage capacitor Cst2 is formed so as to holdthe compensation voltage φ for a predetermined period of time. Thestructure of the pixel P including the sensing TFT T1 and thecompensation TFT T2 is not limited to the structure shown in FIG. 6 andmay variously changed. In the following description, the structure shownin FIG. 6 is used as an example of the structure of the pixel P for thesake of brevity and ease of reading. Each pixel P is connected to thedata line 14, the gate line 15, and the compensation voltage supply lineCL and may be additionally connected to the sensing current supply lineSL if necessary or desired. Each pixel P receives a high potential celldriving voltage VDD and a low potential cell driving voltage VSS from apower generator (not shown).

The timing controller 11 rearranges digital video data RGB received fromthe outside in conformity with a resolution of the display panel 10 andsupplies the rearranged digital video data RGB to the data drivingcircuit 12. The timing controller 11 generates a data control signal DDCfor controlling operation timing of the data driving circuit 12 and agate control signal GDC for controlling operation timing of the gatedriving circuit 13 based on timing signals, such as a vertical syncsignal Vsync, a horizontal sync signal Hsync, a dot clock DCLK, and adata enable signal DE.

The data driving circuit 12 converts the digital video data RGB receivedfrom the timing controller 11 into an analog data voltage based on thedata control signal DDC and supplies the data voltage to the data lines14. The data driving circuit 12 differently generates the compensationvoltage φ based on a sensing current received from the display panel 10and supplies the compensation voltage φ to the compensation voltagesupply lines CL under the control of the timing controller 11. Thecompensation voltage φ compensates for changes in the threshold voltageof the driving TFT DT and varies depending on the threshold voltage ofthe driving TFT DT sensed through the sensing current. The data drivingcircuit 12 may output the compensation voltage φ suitable for a currentthreshold voltage of the driving TFT DT with reference to a previouslydetermined first lookup table, in which the compensation voltage φ isstored depending on the threshold voltage of the driving TFT DT. Thecompensation voltage φ may gradually increase as the threshold voltageof the driving TFT DT is shifted to the right (+). On the contrary, thecompensation voltage φ may gradually decrease as the threshold voltageof the driving TFT DT is shifted to the left (−). Because the shift ofthe threshold voltage of the driving TFT DT is recovered to the originalstate through the compensation voltage φ, a reduction in the drivingcurrent resulting from the shift of the threshold voltage iscompensated.

Further, the embodiment of the invention may cause the timing controller11 to additionally modulate the digital video data RGB supplied to thedata driving circuit 12 depending on an amount of the driving current ofeach pixel P measured in the display panel 10 with reference to apreviously determined second lookup table, in which current compensationdata is stored depending on the driving current, so as to additionallycompensate for the driving current flowing in each pixel P.

The gate driving circuit 13 generates the scan signal SCAN based on thegate control signal GDC. The gate driving circuit 13 supplies the scansignal SCAN to the scan signal supply lines in a line sequential manner.The gate driving circuit 13 may be directly formed on the display panel10 in a GIP (Gate-driver In Panel) manner. The gate driving circuit 13may further generate the sensing control signal SEN to be supplied to agate electrode of the sensing TFT T1 and the compensation control signalCP to be supplied to a gate electrode of the compensation TFT T2 underthe control of the timing controller 11. The gate driving circuit 13 maysupply the sensing control signal SEN to the sensing control signalsupply lines and may supply the compensation control signal CP to thecompensation control signal supply lines.

FIG. 6 illustrates a comparison between a compensation method accordingto the embodiment of the invention and a related art compensationmethod.

As shown in FIG. 6, the related art compensation method sensed a currentIds flowing in the driving TFT DT to sense the shift of the thresholdvoltage of the driving TFT DT and greatly modulated a data voltage Vdataby a compensation voltage corresponding to an increase amount ‘φ’ of thethreshold voltage. Afterwards, the related art compensation methodapplied a modulation voltage (Vdata+φ) to the gate electrode of thedriving TFT DT. Namely, when a current characteristic curve of thedriving TFT DT is shifted to the right due to an increase in thethreshold voltage of the driving TFT DT, the related art compensationmethod increased only a magnitude of a gate-source voltage of thedriving TFT DT while maintaining a shifted state of the currentcharacteristic curve. As a result, according to the related artcompensation method, the degradation of the threshold voltage of thedriving TFT DT was rather accelerated due to the degradationcompensation.

On the other hand, the compensation method according to the embodimentof the invention senses a current Ids flowing in the driving TFT DT tosense the shift of the threshold voltage of the driving TFT DT andapplies the compensation voltage φ corresponding to an increase amountof the threshold voltage to the sub-gate electrode of the driving TFTDT, thereby recovering the shift of the threshold voltage to theoriginal state. Namely, as shown in FIGS. 8A and 8B, when a currentcharacteristic curve of the driving TFT DT is shifted to the right dueto an increase in the threshold voltage of the driving TFT DT, thecompensation method according to the embodiment of the invention againmoves the current characteristic curve of the driving TFT DT to anoriginal position.

For this, each pixel P according to the embodiment of the invention mayinclude the OLED, the driving TFT DT having the double gate structurefor controlling the current Ids flowing in the OLED, the switching TFTST which is turned on or off in response to the scan signal SCAN andapplies the data voltage Vdata to the main gate electrode of the drivingTFT DT, the first storage capacitor Cst1 which is connected between themain gate electrode and the source electrode of the driving TFT DT andstores the data voltage Vdata, the compensation TFT T2 which is turnedon or off in response to the compensation control signal CP and appliesthe compensation voltage φ to the sub-gate electrode of the driving TFTDT, and the second storage capacitor Cst2 which is connected between thesub-gate electrode and the source electrode of the driving TFT DT andstores the compensation voltage 9. Each pixel P according to theembodiment of the invention may further include the sensing TFT T1 whichis turned on or off in, response to the sensing control signal SEN tosense a current flowing in the driving TFT DT and applies the sensedcurrent to the data driving circuit 12.

The OLED is connected between the high potential cell driving voltageVDD and the low potential cell driving voltage VSS. The main gateelectrode of the driving TFT DT is connected to a first node N1, thesub-gate electrode of the driving TFT DT is connected to a third nodeN3, the drain electrode of the driving TFT DT is connected to the highpotential cell driving voltage VDD, and the source electrode of thedriving TFT DT is connected to an anode electrode of the OLED. A gateelectrode of the switching TFT ST is connected to the scan signal supplyline, a drain electrode of the switching TFT ST is connected to the dataline 14, and a source electrode of the switching TFT ST is connected tothe first node N1. A gate electrode of the compensation TFT T2 isconnected to the compensation control signal supply line, a drainelectrode of the compensation TFT T2 is connected to the compensationvoltage supply line CL, and a source electrode of the compensation TFTT2 is connected to the third node N3. A gate electrode of the sensingTFT T1 is connected to the sensing control signal supply line, a drainelectrode of the sensing TFT T1 is connected to a second node N2, and asource electrode of the sensing TFT T1 is connected to the sensingcurrent supply line SL.

FIGS. 7A to 8B illustrate a compensation principle of the thresholdvoltage of the driving TFT DT.

As shown in FIGS. 7A and 7B, the driving TFT DT according to theembodiment of the invention includes a main gate electrode GE1 and asub-gate electrode GE2 which are respectively positioned over and underan active layer with the active layer used to form a current channelinterposed therebetween, and a source electrode SE and a drain electrodeDE which are electrically connected to each other through the activelayer. The data voltage Vdata is applied to the main gate electrode GE1,and the driving current flowing in the current channel is determineddepending on a potential difference between the main gate electrode GE1and the source electrode SE.

As shown in FIG. 7A, when the positive data voltage Vdata is applied tothe main gate electrode GE1 of the driving TFT DT for a long time,electrons (−) are concentrated inside the channel due to a positivestress accumulated on the main gate electrode GE1, and thus a channelresistance increases. Hence, as shown in FIG. 8A, the threshold voltageof the driving TFT DT is shifted from ‘Vth1’ to ‘Vth2’ by ‘φ’, and thecurrent characteristic curve of the driving TFT DT is right shifted from‘A’ to ‘B’. As a result, a current flowing between the drain and sourceelectrodes of the driving TFT DT is reduced from ‘I1’ to ‘I2’ by ‘ΔI’under the same conditions.

In this state, as shown in FIG. 7B, when the compensation voltagecorresponding to ‘φ’ is applied to the sub-gate electrode GE2 of thedriving TFT DT, the electrons (−) inside the channel are distributed,and thus the channel resistance decreases. Hence, as shown in FIG. 8B,the threshold voltage of the driving TFT DT is recovered from ‘Vth2’near to ‘Vth1’, and the current characteristic curve of the driving TFTDT is left shifted from ‘B’ to ‘C’. As a result, the current flowingbetween the drain and source electrodes of the driving TFT DT iscompensated from ‘I2’ to ‘I1’ by ‘ΔI’ under the same conditions.

FIG. 9 illustrates electrical characteristics of the double gate typedriving TFT DT.

As shown in FIG. 9, as a bias voltage applied to the sub-gate electrodeof the double gate type driving TFT DT increases, the electricalcharacteristics of the driving TFT DT gradually change. When the biasvoltage applied to the sub-gate electrode increases to −30V, −20V, −10V,0V, 10V, 20V, and 30V, the threshold voltage and the currentcharacteristic curve of the driving TFT DT are gradually shifted to theleft in proportion to a magnitude of the bias voltage.

FIGS. 10 and 11 illustrate kinds of the double gate type driving TFT DTcapable of performing a bidirectional control.

As shown in FIG. 10, a double gate type driving TFT DT according to theembodiment of the invention includes a sub-gate electrode GE2 formedunder an active layer ACT in a coplanar structure, in which all of amain gate electrode GE1, a source electrode SE, and a drain electrode DEare positioned over the active layer ACT. More specifically, in thedouble gate type driving TFT DT having the coplanar structure, thesub-gate electrode GE2 is formed on a substrate GLS, and a buffer layerBUF is formed between the sub-gate electrode GE2 and the active layerACT. Further, a gate insulating layer GI, the main gate electrode GE1,and an interlayer dielectric layer IL are sequentially formed over theactive layer ACT. A source electrode and a drain electrode are formed sothat they pass through the interlayer dielectric layer IL and the gateinsulating layer GI and are connected to the active layer ACT.

As shown in FIG. 11, a double gate type driving TFT DT according to theembodiment of the invention includes a sub-gate electrode GE2 formedover an active layer ACT in an inverted coplanar structure, in which allof a main gate electrode GE1, a source electrode SE, and a drainelectrode DE are positioned under the active layer ACT. Morespecifically, in the double gate type driving TFT DT having the invertedcoplanar structure, the main gate electrode GE1 and a gate insulatinglayer GI are sequentially formed on a substrate GLS, and the activelayer ACT, the source electrode SE, and the drain electrode DE aresimultaneously formed on the gate insulating layer GI. Further, apassivation layer PASI covering the active layer ACT, the sourceelectrode SE, and the drain electrode DE is formed, and the sub-gateelectrode GE2 is formed on the passivation layer PASI.

FIGS. 12A to 12C sequentially illustrate a method of compensating forthe threshold voltage according to the embodiment of the invention. Morespecifically, FIGS. 12A to 12C illustrate an operation of a pixelfurther including a sensing TFT T1 as an example. All of remainingoperations except a sensing operation of a driving current describedlater are applied to a pixel not including the sensing TFT T1.

As shown in FIG. 12A, the embodiment of the invention turns on theswitching TFT ST and applies the data voltage Vdata to the first storagecapacitor Cst1 connected to the main gate electrode of the driving TFTDT, thereby storing the data voltage Vdata in the first storagecapacitor Cst1. As shown in FIG. 12B, the driving current Ids flowsbetween the drain electrode and the source electrode of the driving TFTDT by a voltage between both terminals of the first storage capacitorCst1, i.e., the gate-source voltage of the driving TFT DT. In thisstate, the embodiment of the invention turns on the sensing TFT T1 andsenses the driving current Ids flowing in the driving TFT DT. Theembodiment of the invention supplies the driving current Ids to the datadriving circuit.

The embodiment of the invention produces the compensation voltagecorresponding to the driving current Ids in the data driving circuit.The embodiment of the invention turns on the compensation TFT T2 andapplies the compensation voltage to the second storage capacitor Cst2connected to the sub-gate electrode of the driving TFT DT, therebystoring the compensation voltage in the second storage capacitor Cst2.The shift of the threshold voltage resulting from the gate bias stressapplied to the main gate electrode is recovered by the compensationvoltage applied to the sub-gate electrode.

As described above, the embodiment of the invention includes the doublegate type driving TFT having the two gate electrodes and applies thecompensation voltage corresponding to a change amount of the thresholdvoltage of the driving TFT to the sub-gate electrode of the driving TFT,thereby recovering the shift of the threshold voltage to the originalstate. Hence, the embodiment of the invention solves the related artproblems of the acceleration of the degradation in the compensationprocess and a limitation of the compensation range. The embodiment ofthe invention efficiently compensates for the degradation of thethreshold voltage, thereby preventing a drive failure resulting from along drive and improving the reliability. Further, the embodiment of theinvention increases the uniformity of the luminance and greatlyincreases the life span of the products.

Although embodiments have been described with reference to a number ofillustrative embodiments thereof, it should be understood that numerousother modifications and embodiments can be devised by those skilled inthe art that will fall within the scope of the principles of thisdisclosure. More particularly, various variations and modifications arepossible in the component parts and/or arrangements of the subjectcombination arrangement within the scope of the disclosure, the drawingsand the appended claims. In addition to variations and modifications inthe component parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. An organic light emitting display comprising: adisplay panel including a plurality of pixels to display an image; and adata driving circuit configured to output data voltage corresponding tothe image, wherein each of the plurality of pixels includes: an organiclight emitting diode; a first thin film transistor (TFT) having a doublegate structure including a main gate electrode connected to a first nodeand a sub-gate electrode connected to a third node; a second TFTconfigured to apply the data voltage to the main gate electrode of thefirst TFT; a first storage capacitor connected to the first node and asecond node between the organic light emitting diode and the first TFT;a second storage capacitor connected to the third node and the secondnode; and a third TFT connected to the sub-gate electrode of the firstTFT, wherein at least one of the plurality of pixels further includes afourth TFT connected to the second node, and wherein the second storagecapacitor has a first terminal connected to the sub-gate of the firstTFT and the third TFT, and a second terminal directly connected to thefourth TFT and the organic light emitting diode.
 2. The organic lightemitting display of claim 1, wherein the fourth TFT is included in eachpixel group having at least two pixels.
 3. The organic light emittingdisplay of claim 1, wherein the data driving circuit senses a thresholdvoltage of the first TFT through a sensed driving current and outputs acompensation voltage corresponding to the sensed threshold voltage withreference to a first lookup table, in which the compensation voltage isstored depending on the threshold voltage.
 4. The organic light emittingdisplay of claim 3, wherein the compensation voltage gradually increasesas the threshold voltage of the first TFT is shifted to a positivedirection, and gradually decreases as the threshold voltage of the firstTFT is shifted to a negative direction.
 5. The organic light emittingdisplay of claim 1, wherein the first TFT is implemented as a coplanarstructure, in which all of the main gate electrode, a source electrode,and a drain electrode are positioned over an active layer, wherein thesub-gate electrode is formed under the active layer.
 6. The organiclight emitting display of claim 1, wherein the first TFT is implementedas an inverted coplanar structure, in which all of the main gateelectrode, a source electrode, and a drain electrode are positionedunder an active layer, wherein the sub-gate electrode is formed over theactive layer.
 7. The organic light emitting display of claim 1, furthercomprising a timing controller configured to modulate digital video datasupplied to the data driving circuit depending on an amount of a drivingcurrent of each pixel measured in the display panel with reference to asecond lookup table, in which current compensation data is previouslystored depending on the driving current.
 8. The organic light emittingdisplay of claim 1, wherein the first TFT is a driving TFT, the secondTFT is a switching TFT, the third TFT is a compensation TFT, and thefourth TFT is a sensing TFT.